1. Field of the Invention
The present invention relates to a semiconductor element, a manufacturing method thereof, and a high frequency integrated circuit using the semiconductor element.
2. Description of the Related Art
A device using a microwave has recently been in widespread use in applications such as mobile communications. The development of microwave integrated circuits (MICs) each installed in the device has been pursued. Of the MICs, a monolithic MIC (MMIC) in which active and passive elements are simultaneously fabricated and built onto a semiconductor substrate, has the advantage that since the monolithic MIC can be manufactured massively and uniformly according to a semiconductor process, it is excellent in productivity and reproducibility and easy to make its reductions in size and weight, for example.
Inductor elements (hereinafter might be called simply “inductors”) are components important to an electric circuit and might be often essential thereto in view of its configuration. Since a spiral inductor of the inductors can be integrated into MMIC, it is of a general element as an inductor fabricated on a semiconductor substrate.
A prior art example of a spiral inductor will be explained with reference to FIG. 21. FIG. 21(A) is a schematic plan view of the conventional spiral inductor provided on an SOI substrate as viewed from its upper side. FIG. 21(B) is a cross-sectional view taken along line A-A shown in FIG. 21(A). FIG. 21(C) is a cross-sectional view taken along line C-C shown in FIG. 21(A). An insulative buried oxide film 215 exists between a support substrate 213 and an insulating layer 216. Electrodes for the spiral inductor 211 are formed in the insulating layer 216. The electrodes for the spiral inductor 211 comprise a first input/output electrode 231, a spiral electrode 221, a spiral electrode lead-out electrode 243 and a second input/output electrode 233 electrically connected in this order. The spiral electrode 221 is shaped in a turbinated spiral form.
The spiral inductor is used to adjust or control the impedance of an input/output section of a high frequency circuit, for example (refer to, for example, a patent document (Japanese Unexamined Patent Publication No. 2002-124638 (sections 0024 through 0026 and FIG. 1)). In the high frequency circuit, the accuracy of such impedance control exerts a great influence on the characteristic of the whole high frequency circuit.
In order to obtain the characteristic of the spiral inductor 211 by simulation, it is possible to carry out three-dimensional simulation from the structure of such a spiral inductor 211 as shown in FIG. 21. The three-dimensional simulation, however, needs enormous amounts of time. In view of this point, a simplified equivalent circuit is normally made up and circuit simulation is effected on the equivalent circuit. An equivalent circuit model of the spiral inductor for circuit simulation is shown in FIG. 22. In FIG. 22, Rs21, Ls21 and Cs21 indicate the value of a resistance of the spiral electrode 221, the value of an inductance thereof and the value of a capacitance thereof, respectively. Cins21 and Cins22 indicate the values of capacitances of the insulating layer 216, Cox21 and Cox22 indicate the values of capacitances of the buried oxide film 215, Csub21 and Csub22 indicate the values of capacitances of the support substrate 213, and Rsub21 and Rsub22 indicate the values of resistances of the support substrate 213, respectively. Incidentally, it is considered that a plurality of paths connect among the electrodes constituting the spiral inductor 211 and a ground point (called GND) so as to extend from respective portions of the electrodes constituting the spiral inductor 211 to GND.
FIG. 22 shows an equivalent circuit model connected to the substrate 213 from two points between the first input/output electrode 231 and the spiral electrode 221 and between the second input/output electrode 233 and the spiral electrode 221.
The value Ls21 of the inductance of the spiral inductor 211 is generally determined depending on the size and winding number or the like of the spiral electrode 221. However, when the whole spiral inductor 211 is considered, the value of the inductance thereof is affected by the inductances of internal and external wirings of the spiral inductor 211, which are connected to the spiral electrode 221, for example. Thus, the value Ls21 of the inductance has a certain degree of deviation.
Therefore, there has been proposed such a variable spiral inductor that the value Ls21 of the inductance can be adjusted after the formation of the spiral inductor 211 (refer to, for example, each of patent documents 2 (Japanese Unexamined Patent Publication No. 2001-291615), 3 (Japanese Unexamined Patent Publication No. 2001-291616) and 4 (Japanese Unexamined Patent Publication No. 2003-179146)).
A prior art example of a variable spiral inductor will be explained with reference to FIGS. 23 and 24 (refer to the patent document 2). FIG. 23 shows an equivalent circuit model of the variable spiral inductor. The equivalent circuit shown in FIG. 23 is different from the equivalent circuit shown in FIG. 22 only in components that constitute a spiral electrode 321. Other components are identical in configuration to those described with reference to FIG. 22. The variable spiral inductor is a circuit which adjusts or controls the value Rs31 of a resistance of the spiral electrode 321, the value Ls31 of its inductance and the value Cs31 of its capacitance.
FIG. 24 is a schematic plan view of the variable spiral inductor 311 as viewed from its upper side. Since its sectional view is similar to FIG. 21(B), the illustration of its section and the description thereof will be omitted. Electrodes for the variable spiral inductor 311 comprise a first input/output electrode 331, a first spiral electrode 321a, a trimming section 323, a second spiral electrode 321b and a second input/output electrode 333 electrically connected in this order. The first spiral electrode 321a, the trimming section 323 and the second spiral electrode 321b constitute a spiral electrode 321. The trimming section 323 comprises trimming electrodes 323a through 323e. The trimming electrodes 323a through 323e are respectively formed so as to connect the first spiral electrode 321a and the second spiral electrode 321b. The trimming electrodes 323a through 323e are cut one by one in order by irradiation thereof with a laser beam or the like to thereby change the value of the inductance of the variable spiral inductor 311.
However, the variable spiral inductor according to the above conventional example takes time and increases in cost because the value of its inductance is adjusted by a physical method that each electrode is cut, and encounters difficulties in adjusting the inductance with high accuracy. In order to control or adjust the inductance with high accuracy, it is important to fabricate a variable spiral inductor adjustable by voltage control without depending on physical control and build it onto an integrated circuit. However, an easy inductance control method has not yet proposed so far. Therefore, the application of the inductor to a receiving circuit section and a transmitting circuit section of a high frequency circuit is also limited. For example, the receiving circuit section of the high frequency circuit finds difficulty impedance-matching of input and output signals. Also the transmitting circuit section of the high frequency circuit finds it difficult to broaden a variable range of an oscillation frequency.